Passive immunotherapy for mouse leukemias with antisera of "directed" specificity: synergism with the action of cyclophosphamide.

Antileukemia sera with "directed" specificity are produced by immunization of rabbits with mouse leukemia cells admixed with normal antigen blocking (NAB) serum. Addition of NAB serum to the leukemia cells inhibits production of antibodies to normal cell components and directs specificity toward leukemia cell antigens. The resulting antileukemia serum (ALK-NABS) was not sufficiently potent to produce more than moderate therapy in the standard L1210 leukemia therapy assay. When given together with noncurative doses of cyclophosphamide (CTX), ALK-NABS acts synergistically. It is most effective when given early after injection of the leukemia cells and prior to injection of CTX. Daily repeated injections of a given dose are more effective than a single injection of that dose. Most important, small doses of ALK-NABS produce a significant prolongation of lifespan in conjunction with CTX. Results of therapy for BW-A leukemia with ALK-NABS in conjunction with CTX were negative.